the insects structure and function

The head has a pair of long antennae, large contiguous compound eyes and threewell-developed ocelli single-faceted, simple eyes.. The headhas a pair of short bristle-like antennae, a pai

The Insects Structure and Function

F I F T H E D I T I O N

The Insects has been the standard textbook in the field since the first edition waspublished over 40 years ago Building on the strengths of Chapman’s original text,this long-awaited new edition has been revised and expanded by a team of eminentinsect physiologists, bringing it fully up to date for the molecular era

The chapters retain the successful structure of the earlier editions, focusing onparticular functional systems rather than on taxonomic groups and making it easy forstudents to delve into topics without extensive knowledge of taxonomy The focus is

on form and function, bringing together basic anatomy and physiology andexamining how these relate to behavior This, combined with nearly 600 clearillustrations, provides a comprehensive understanding of how insects work

Now also featuring a richly illustrated prologue by George McGavin, this is anessential text for students, researchers and applied entomologists alike

R F Chapman (1930–2003) was an eminent insect physiologist and Professor in theDivision of Neurobiology at the University of Arizona His first four editions of TheInsects have formed the standard text in the field for more than 40 years

Stephen J Simpson is ARC Laureate Fellow in the School of Biological Sciences andAcademic Director of the Perkins Centre for the study of obesity, diabetes andcardiovascular disease at the University of Sydney His core research aims are tounderstand swarming in locusts and to develop and implement an integrativeframework for studying nutrition In 2012 he was awarded the Wigglesworth Medalfrom the Royal Entomological Society of London

Angela E Douglas is Daljit S and Elaine Sarkaria Professor of Insect Physiology andToxicology at Cornell University, New York Her research and teaching is motivated

by the mechanisms underlying insect function, and her core research interests are theoverlapping topics of insect nutrition and interactions between insects and beneficialmicroorganisms She is a Fellow of The Royal Entomological Society and TheEntomological Society of America

C A M B R I D G E U N I V E R S I T Y P R E S S

Cambridge, New York, Melbourne, Madrid, Cape Town,

Singapore, Sa˜o Paulo, Delhi, Mexico City

Cambridge University Press

The Edinburgh Building, Cambridge CB2 8RU, UK

Published in the United States of America by Cambridge University Press, New York

www.cambridge.org

Information on this title: www.cambridge.org/9780521113892

© Cambridge University Press 1998, 2013

This publication is in copyright Subject to statutory exception

and to the provisions of relevant collective licensing agreements,

no reproduction of any part may take place without

the written permission of Cambridge University Press.

First published by Edward Arnold 1969

Second edition 1971, 6th printing 1980

Third edition 1982, 5th printing 1991

Fourth edition published by Cambridge University Press 1998, 7th printing 2011

Fifth edition 2013

Printed in the United Kingdom by the MPG Books Group

A catalogue record for this publication is available from the British Library

Library of Congress Cataloguing in Publication data

Chapman, R F (Reginald Frederick)

The insects : structure and function / R F Chapman – 5th edition / edited by

Stephen J Simpson, Angela E Douglas.

Cambridge University Press has no responsibility for the persistence or

accuracy of URLs for external or third-party internet websites referred to

in this publication, and does not guarantee that any content on such

websites is, or will remain, accurate or appropriate.

Part I The head, ingestion, utilization

and distribution of food

Introduction 814.1 Required nutrients 824.2 Balance of nutrients 874.3 Nutritional effects on growth, development,reproduction and lifespan 95

4.4 Contribution of symbiotic microorganisms

to insect nutrition 98Summary 104

5 Circulatory system, blood and the

Introduction 1075.1 The circulatory system 1085.2 Circulation 113

5.3 Hemolymph 1175.4 Hemocytes 124Summary 129

Introduction 1326.1 Fat body structure and development 1336.2 Storage and utilization of energy

and nutrients 1376.3 Function as an endocrine organ andnutritional sensor 142

Summary 144

Part II The thorax and locomotion

Introduction 1497.1 Segmentation of the thorax 1507.2 Morphology of the thorax 151

7.3 Muscles of the thorax 155

Summary 155

Introduction 157

8.1 Structure of the legs 158

8.2 Walking and running 166

8.3 Other mechanisms of terrestrial

locomotion 1738.4 Aquatic locomotion 180

8.5 Other uses of legs 186

Summary 189

Introduction 193

9.1 Structure of the wings 194

9.2 Form of the wings 204

9.3 Movement of the wings 207

9.4 Wing kinematics 214

9.5 Aerodynamic mechanisms 221

9.6 Power for flight 223

9.7 Sensory systems for flight control 225

10.3 Regulation of muscle contraction 244

10.4 Energetics of muscle contraction 252

10.5 Muscular control in the intact insect 254

10.6 Changes during development 257

12.2 Spermatozoa 28612.3 Transfer of sperm tothe female 29212.4 Other effects of mating 306Summary 310

Introduction 31313.1 Anatomy of the internal reproductiveorgans 314

13.2 Oogenesis 31713.3 Ovulation 33313.4 Fertilization of the egg 33313.5 Oviposition 335

Summary 343

Introduction 34714.1 The egg 34814.2 Embryogenesis 35714.3 Alternative strategies of acquiring nutrients

by embryos 37914.4 Sex determination 38814.5 Parthenogenesis 39014.6 Pedogenesis 392Summary 393

Introduction 39815.1 Hatching 39915.2 Larval development 40315.3 Metamorphosis 41715.4 Control of postembryonicdevelopment 43615.5 Polyphenism 44315.6 Diapause 448Summary 454

Part IV The integument, gas exchange

and homeostasis

Introduction 46316.1 Epidermis 46416.2 The cuticle 46916.3 Chemical composition of the cuticle 47316.4 Types of cuticles 483

16.5 Molting 48816.6 Cuticle formation 49316.7 Functions of the integument 497Summary 498

Introduction 50117.1 Tracheal system 50217.2 Spiracles 51117.3 Cutaneous gas exchange 51517.4 Respiratory pigments 51517.5 Gaseous exchange in terrestrial insects 51617.6 Gaseous exchange in aquatic insects 52817.7 Insects subject to occasional submersion 53717.8 Gas exchange in endoparasitic insects 54017.9 Other functions of the tracheal system 54117.10 Gas exchange in insect eggs 542Summary 542

Introduction 54618.1 Excretory system 54718.2 Urine production 55218.3 Modification of the primary urine 55518.4 Control of diuresis 559

18.5 Nitrogenous excretion 56218.6 Detoxification 56718.7 Non-excretory functions of the Malpighiantubules 569

18.8 Nephrocytes 57118.9 Water regulation 573Summary 584

Introduction 58819.1 Body temperature 58919.2 Thermoregulation 59519.3 Performance curves 59819.4 Behavior and survival at lowtemperatures 600

19.5 Activity and survival at hightemperatures 607

19.6 Acclimation 61019.7 Cryptobiosis 61119.8 Temperature and humidity receptors 61119.9 Temperature-related changes in thenervous system 614

19.10 Large-scale patterns in insectthermal biology 616Summary 617

20.5 Controlling behavior 659Summary 669

Introduction 67421.1 Chemical structure of hormones 67521.2 Endocrine organs 684

21.3 Transport of hormones 69121.4 Regulation of hormone titer 69121.5 Mode of action of hormones 696Summary 703

B Perception of the environment

Introduction 708

22.1 Compound eyes 709

22.2 Form and motion vision 715

22.3 Receptor physiology, color and

polarization vision 72122.4 Dorsal ocelli 731

22.5 Stemmata 732

22.6 Other visual receptors 734

22.7 Magnetic sensitivity and photoreception 735

24.3 Distribution and numbers of sensory

sensilla 77624.4 How the chemosensory sensillum

functions 77624.5 Integrating function and behavior 788

24.6 Projections to the central nervous system 789

Summary 791

25 Visual signals: color and light

26 Mechanical communication: producing

Introduction 82426.1 Nature and transmission of acoustic andvibrational signals 825

26.2 Significance of acoustic and vibrationalsignals 826

26.3 Mechanisms producing sounds andvibrations 832

26.4 Patterns of acoustic and vibrationalsignals 845

26.5 Neural regulation of sound production 847Summary 853

27 Chemical communication: pheromones

Introduction 85727.1 Defining chemical signals 85827.2 Pheromones used in intraspecificcommunication 858

27.3 Information content of pheromonalsignals 874

27.4 Biosynthesis of pheromones 87627.5 Regulation of pheromone production 88227.6 Perception of pheromones and otherinfochemicals 883

27.7 Information transfer between species:allelochemicals 885

27.8 Producing, storing and releasingallomones 887

27.9 Allelochemicals used in defense 89027.10 Mimicry 895

Summary 898

C O N T R I B U T O R S

Lars Chittka

School of Biological and Chemical Sciences

Queen Mary, University of London

UK

Bronwen W Cribb

Centre for Microscopy & Microanalysis and

School of Biological Sciences

The University of Queensland, Brisbane

Institute of Molecular Cell and Systems Biology

College of Medical, Veterinary & Life Sciences

University of Glasgow

UK

Jon F Harrison

School of Life Sciences

Arizona State University, AZ

Division Kidney, Urologic & Hematologic Diseases

NIDDK, National Institutes of Health

Bethesda, MD

USA

Michael F LandSchool of Life SciencesUniversity of Sussex, Brighton UKTom Matheson

Department of BiologyUniversity of LeicesterUK

George C McGavinOxford University Museum of Natural HistoryOxford

UKJeremy McNeilDepartment of BiologyUniversity of Western Ontario, LondonCanada

David J MerrittSchool of Biological SciencesThe University of Queensland, BrisbaneAustralia

Hans MerzendorferFachbereich Biologie/Chemie, Osnabru¨ckGermany

Jocelyn G MillarDepartment of EntomologyUniversity of California, RiversideUSA

Stuart ReynoldsDepartment of Biology & BiochemistryUniversity of Bath

UK

Centre for Evolutionary Biology

School of Animal Biology

The University of Western Australia, Crawley

Australia

Stephen J Simpson

School of Biological Sciences

The University of Sydney

John S TerblancheDepartment of Conservation Ecology & EntomologyFaculty of AgriSciences

Stellenbosch UniversitySouth Africa

Peter VukusicSchool of PhysicsUniversity of ExeterUK

Lutz T WasserthalInstitut fu¨r Zoologie IUniversita¨t Erlangen-Nu¨rnbergGermany

P R E F A C E

Reginald Chapman’s The Insects: Structure and Function has been thepreeminent textbook for insect physiologists for the past 43 years (since themoon landing, in fact) For generations of students, teachers and researchers TheInsects has provided the conceptual framework explaining how insects work.Without this book, the lives of entomologists worldwide would have beensubstantially more difficult Nevertheless, the most recent (fourth) edition of thisremarkable book was published in 1998, and a great deal has happened since then.Sadly, Reg died in 2003 and there was no reasonable prospect of any other persontaking on the next revision single-handed We have decided to take a differentapproach: to invite a team of eminent insect physiologists to bring their expertise

to the collective enterprise of writing the fifth edition of The Insects

Our aim has been to protect the identity of The Insects by workingwith Reg’s original text Certain areas have needed more revision than others,and some sections have been shrunk to accommodate advances in others Oursole major deviation from the style of previous editions has been to remove allcitations to primary literature from the main text These in-text citations hadaccreted across successive revisions, and were somewhat patchy in coveragethroughout the book With the availability of online literature search enginestoday, students and researchers alike are better served by a short list of keyreferences at the end of each chapter to provide a lead-in to the literature

It has been the greatest pleasure for us to work with 23 colleaguesfrom seven countries over the last four years, as the fifth edition of The Insectshas taken shape This project brings into sharp relief the intellectual strength andvigor of our discipline – the new discoveries over the last 14 years since the fourthedition are nothing short of breathtaking We have also come to admire, more thanever, the breadth of Reg’s knowledge and understanding of insects He was aremarkable man

S T E V E S I M P S O N and A N G E L A D O U G L A S

A C K N O W L E D G M E N T S

We wish to express our considerable gratitude to all our authors for their insight,expertise and commitment to this venture We also thank Pedro Telleria-Teixeirafor his tireless efforts in helping prepare the manuscript for submission, and toCambridge University Press for taking it from there Finally, we thank ElizabethBernays for her encouragement to take on the task We hope that Reg would havebeen pleased with the result

P R O L O G U E

G E O R G E C M c G A V I N

The ancestor of the Arthropoda was in all probability a segmented worm-likemarine creature that lived in oceans during the late Precambrian By the early tomid-Cambrian (540–520 million years ago) the early arthropods had alreadyevolved into a range of clearly recognizable groups with distinct body plans.Arthropods are characterized by a number of features: the possession of aperiodically molted, chitinous cuticle that acts as a rigid exoskeleton for theinternal attachment of striated muscles; segmental paired legs; and the aggregationand/or fusing of body segments into discrete functional units, of which the mostuniversal is the head Besides the head there may be a trunk, as in the Myriapoda, or

a separate thorax and abdomen as in the Crustacea and Hexapoda

Based on the ubiquity ofa-chitin in arthropod cuticles, similarities inmusculature and tendon systems and recent molecular data, the overwhelmingconsensus of opinion is that this very large taxon is monophyletic However, therelationships within the Arthropoda have been the subject of much controversyfor more than 100 years Recent molecular and genetic data confirm that theHexapoda (comprising the Insecta and three other non-insect hexapod classes)are monophyletic, but that Crustacea are not The monophyletic Hexapoda andparaphyletic Crustacea are now thought to form a single superclade called thePancrustacea (Fig 1) The mandibles of these two groups have similar origins, andthe development of the nervous system is similar, as is the structure and wiring ofthe compound eyes

A little over 1.5 million species of living organism have been scientificallydescribed to date The vast majority (66%) are arthropods such as crustaceans,arachnids, myriapods and insects Insects represent 75% of all animals, and oneinsect order – the beetles (Coleoptera) – is famously species-rich, but anothercomprising the wasps, bees and ants (Hymenoptera) may rival the beetles iftaxonomists ever complete their studies One thing is clear, however – the full extent

of Earth’s biodiversity remains a mystery From attempts over 30 years ago toestimate the number of extant species to the present day we still only have a roughidea of how many species live alongside us Estimates range from as few as fivemillion to perhaps as many as 10–12 million species The task of enumerating themmay become substantially easier as the loss and degradation of natural habitats,especially the forests of the humid tropics, continues unabated It is certain that themajority of insect species will become extinct before they are known to science

Mandibulata

Pancrustacea

Altocrustacea Miracrustacea

Mystacocarida Pentastomida

Ostracoda Branchiura

Acari Scorpiones Xiphosura

Chilopoda

Collembola Archaeognatha

Remipedia

Cephalocarida Thysanura

Tardigrada

Onychophora Ephemeroptera

0.03

Diplura

Figure 1 Phylogram

of relationships for 75 arthropod and five outgroup species Reprinted by permission from Macmillan Publishers Ltd: Regier, J C., Shultz,

J W., Zwick, A., Hussey, A., Ball, B., Wetzer, R., Martin, J W and Cunningham, C W (2010) Arthropod relationships revealed

by phylogenomic analysis of nuclear protein-coding sequences Nature 463, 1079–1083.

Insects are the dominant multicellular life form on the planet, ranging

in size from minute parasitic wasps at around 0.2 mm to stick insectsmeasuring 35 cm in length Insects have evolved diverse lifestyles and althoughthey are mainly terrestrial, there are a significant number of aquatic species.Insects have a versatile, lightweight and waterproof cuticle, are generally small

in size and have a complex nervous system surrounded by an effective blood–brain barrier Insects were the first creatures to take to the air and haveprodigious reproductive rates These factors, together with the complexinteractions they have with other organisms, have led to their great success both

in terms of species richness and abundance The very high diversity of insectstoday is the result of a combination of high rates of speciation and the fact thatmany insect taxa are persistent – that is, they show relatively low rates ofextinction

In comparison to insects, vertebrate species make up less than 3% of all species

As herbivores they are altogether out-munched by the myriad herbivorous insects

In tropical forests, for example, 12–15% of the total leaf area is eaten by insects ascompared with only 2–3% lost to vertebrate herbivores Termites remove moreplant material from the African savannahs than all the teeming herds of wildebeestand other ungulates put together Vertebrates also fail to impress as predators Antsare the major carnivores on the planet, devouring more animal tissue per annumthan all the other carnivores In many habitats ants make up one-quarter of thetotal animal biomass present

Insects pollinate the vast majority of the world’s 250 000 or so species offlowering plant The origin of bees coincides with the main radiation of theangiosperms approximately 100 million years ago, and without them therewould be no flowers, fruit or vegetables At least 25% of all insect species areparasites or predators of other insect species Insects are also important in nutrientrecycling by disposing of carcasses and dung

Insects are the principal food source for many other animals Virtually all birdsand a large number of other vertebrates feed on them An average brood of great titchicks will consume around 120 000 caterpillars while they are in the nest and asingle swallow chick may consume upwards of 200 000 bugs, flies and beetlesbefore it fledges

Insects can also have a huge negative impact on humans One-sixth of allcrops grown worldwide are lost to herbivorous insects and the plant diseases theytransmit About one in six human beings alive today is affected by an insect-borneillness such as plague, sleeping sickness, river blindness, yellow fever, filariasisand leishmaniasis About 40% of the world’s population are at risk of malaria.More than 500 million people become severely ill and more than one million diefrom this disease every year To complete the destructive side of their activities,insects can cause great damage to wooden structures and a wide range of naturalmaterials and fabrics

But without insects performing essential ecosystems services, the Earth would be

a very different place and most terrestrial vertebrates that depend on them directly

as food would become extinct The loss of bees alone might cause the extinction ofone-quarter of all life on Earth A total loss of insects would see the humanpopulation plummet to perhaps a few hundred thousand individuals subsistingmainly on cereals

Their small size and high reproductive rates make insects ideal model systems inmolecular, cellular, organismal, ecological and evolutionary studies Indeed, many

of the most important discoveries in genetics, physiology, behavior, ecology andevolutionary biology have relied on insects

There may come a day when humans venture far enough into space to visit otherplanets on which life has developed If we do and there are multicellular organismspresent, it is likely they will look a lot like insects

Mini-biographies of the insect orders

The Insecta and three other classes, the Protura, Diplura and Collembola, togethercomprise the arthropod superclass, Hexapoda The Class Insecta is divided into

30 orders, which are outlined below

Bristletails are the most primitive living insects, having persisted for more than

400 million years They are mainly nocturnal, living in leaf litter and under stones

in a wide range of habitats from coastal to mountainous regions The body, which iselongate with a cylindrical cross-section, is covered in tiny scales and has acharacteristically humped thorax

The head has a pair of long antennae, large contiguous compound eyes and threewell-developed ocelli (single-faceted, simple eyes) The mouthparts are simple, withlong maxillary palps The mandibles have a single point of articulation (termedmonocondylar) and are used to pick at lichens and algae This jaw articulation is avery primitive feature separating the bristletails from all other insects, includingthe Thysanura, which have two points of articulation (dicondylar)

The abdomen has accessory walking appendages called styles (present onabdominal segments 2–9), which support the abdomen when bristletails run overuneven or steep surfaces Surface water can be absorbed through one or two pairs

of eversible vesicles located on the underside of abdominal segments 1–7 Theabdomen has a pair of multi-segmented cerci and a much longer central filament.Bristletails can jump by rapid flexion of the abdomen

T H Y S A N U R A ( Z Y G E N T O M A )

 <400 species

Although very similar to bristletails, silverfish are actually more closely related

to the winged insects The body, which may have a covering of scales, is rathermore flattened and the thorax is not humped Silverfish are scavengers in soil, leaflitter, on trees and sometimes in buildings, where they can be minor pests

The head has a pair of long antennae, small compound eyes and may have ocelli.The maxillary palps are shorter than those of the Archaeognatha and the jaws,although still of a primitive design, have two points of articulation and act in thetransverse plane

Styles may be present on abdominal segments 2–9, but usually on fewer segments(7–9) Pairs of water-absorbing, eversible vesicles usually occur on the abdominalsegments (2–7), although in some species they are absent The end of the abdomenhas a pair of cerci and a central filament Silverfish are fast running but do not jump

T H E W I NG E D IN S E C T S

The infraclass Pterygota is made up of three very unequal divisions The mayflies(Ephemeroptera), comprising<0.3% of all insects species, and the dragonfliesand damselflies (Odonata), comprising~0.5% of all insect species, are each adivision Species in these two divisions are unable to fold their wings back alongthe body Together they are sometimes termed the Paleoptera, although this is not anatural (monophyletic) grouping The third, and by far the largest division,comprising all other insect species, is the Neoptera, which are monophyletic

The order is divided into two suborders, the Schistonota (split-back mayflies) andthe Pannota (fused-back mayflies) Schistonotan nymphs have their wing padsfree along the midline, whereas in pannotan nymphs, the wing pads are fused alongthe midline of the body

Mayflies are soft-bodied with nearly cylindrical bodies, longish legs andtypically two pairs of wings, which, when at rest, are held over the body The headhas a pair of short bristle-like antennae, a pair of large compound eyes and threeocelli Adults have reduced non-functional mouthparts The end of the abdomenbears a pair of elongate cerci and, usually, a single, long central filament.The lifecycle is dominated by the aquatic, nymphal stages and adults live for avery short time, often less than a day.

be rapidly extended Spine-like palps on the labium impale prey items and themask is then folded back toward the mouth

The order is split into two major suborders, the dragonflies (Anisoptera) andthe damselflies (Zygoptera) A third suborder (Ansiozygoptera) comprises only twoOriental species Dragonflies have round heads and very large eyes, while

damselflies have broader heads with widely separated eyes The large eyes giveodonates near all-round vision and, as would be expected of aerial hunters, they areable to resolve distant objects better than any other insect

D I V I S I O N I I I : N E O P T E R A

In all neopterans, flexor muscles attached to a third axillary sclerite at the base ofthe wings allow the wings to be folded back along the body The evolution of awing-folding mechanism allowed much better exploitation of the terrestrialenvironment without the risk of wing damage

Stoneflies are slender insects with soft, slightly flattened bodies The head hasbulging eyes, two or three ocelli and thread-like antennae The mouthparts areweakly developed or non-functional They have two pairs of membranous wings,which are held flat or folded around the body at rest They are not strong fliersand seldom travel far from water The elongate abdomen has a pair of single- ormulti-segmented cerci.

The order is divided into two suborders, the Arctoperlaria and theAntarctoperlaria With the exception of one family, all Arctoperlaria are found inthe Northern Hemisphere All families in the Antarctoperlaria are found in theSouthern Hemisphere

Stonefly nymphs are aquatic and can swim using lateral body movements

Many graze algae from rocks

“tegmina” to cover the larger, membranous hindwings The abdomen carries a pair

of one- or multi-segmented cerci Eggs are typically laid in a toughened case orootheca, a feature shared with the closely related, but entirely predatory Mantodea.The vast majority of cockroaches are nocturnal, omnivorous or saprophagousspecies living in soil and leaf litter communities Only about 40 species areconsidered pests because of their close association with humans, and only half ofthese have a significant impact The main problem is that they can carry a hugediversity of pathogenic organisms on their tarsi and other body parts When theyfeed they regurgitate partly digested food and leave behind their feces and acharacteristic offensive odor Exposure to high levels of cockroach allergens inhouse dust can produce serious health problems such as allergies, dermatitis,eczema and asthma

True binocular vision allows mantids to calculate the distance of their preyusing triangulation The coxa of the front legs is very elongate and the femur isenlarged and equipped with rows of sharp spines and teeth The tibia, which is alsospined or toothed, folds back on the inner face of the femur like a jack knife Thestrike, which takes place in two phases, lasts less than 100 milliseconds In theinitial phase, the tibiae are fully extended in readiness for the second phase, whichtakes the form of a rapid sweeping action The femora are quickly extended and,

at the same time, the tibiae are flexed around the prey

Mantids, which are mainly diurnal, predate a wide range of insects, spiders andother arthropods, which they ambush or stalk Larger species have even beenrecorded catching and eating vertebrates such as frogs, mice and even small birds

I S O P T E R A

 <3000 species

Generally pale and soft-bodied, termites are social insects living in permanentcolonies with different castes of both sexes Workers and soldiers are wingless,while the reproductives (kings and queens) have two pairs of equal-sized wings,which are shed after a nuptial flight

The foodstuff of termites, cellulose, is an abundant biomolecule but is difficult tobreak down Termites have evolved symbiotic relationships with cellulase-producingmicroorganisms to make use of this resource The gut of lower termites harborsprotists, while those of the higher termites (Termitidae) contain bacterial symbionts.Termites can build impressively large nest structures, including the largemulti-vented chimneys that ventilate the subterranean nests of AfricanMacrotermes species and the wedge-shaped nests in northern Australia made

by the magnetic termite, Amitermes meridionalis

have an immense impact on soil enrichment and carbon cycling They mayconsume up to one-third of the annual production of dead wood, leaves and grassand be present in huge numbers, comprising 10% of all animal biomass present

G R Y L L O B L A T T O D E A ( N O T O P T E R A )

 26 species (1 family: Grylloblattidae)

These slender, wingless, slightly hairy insects were first discovered in the CanadianRockies in 1913 and are a relict group confined to certain high-altitude regions acrossthe Northern Hemisphere The head has small compound eyes, although these aresometimes absent, no ocelli, slender, thread-like antennae and simple, chewingmouthparts The abdomen is cylindrical, with a pair of slender, multi-segmented cerci

Grylloblattids live under stones, decaying wood and leaf litter in coldtemperate forests and sometimes in caves There may be eight nymphal instars andcomplete nymphal development may take up to 5–6 years As nymphs get olderthey become darker colored and add segments to their antennae at each molt Theadults typically live for less than two years.

M A N T O P H A S M A T O D E A

 Gladiators, African rock crawlers or heel-walkers

Discovered in 2002, the species that make up this small order live in dry,rocky habitats in southern Africa and may be related to the Grylloblattodea Thehead has well-developed compound eyes and long, slender antennae and bitingmouthparts, but lacks ocelli The name “heel-walkers” refers to the way the clawsare held clear of the ground when walking

These elongate, wingless insects can be found under stone and among tufts

of grasses and other plants At night they emerge to catch other insects, holdingsmall prey using their spiny front and middle legs

of their distinctive abdominal forcep-like cerci The head, which may have a pair

of compound eyes but no ocelli, has biting mouthparts and long antennae Thefront wings are short, leathery and veinless, covering the large, semicircularhindwings

The order is divided into three very unequal suborders The largest – whichaccounts for 99% of all known species – is the Forficulina, which prefer confined,humid microhabitats such as soil, leaf litter or beneath bark The Hemimerina ismade up of 11 species of African cockroach-like earwigs, which are ectoparasites inthe fur of giant rats The Arixenina comprises five blind, wingless South Asianspecies that feed on skin fragments and excreta in the fur or roosts of two species ofmolossid bat

The terminal forceps, which are usually straight in females and curved in males,are used in a variety of ways but mainly as weapons for defense and prey handling,but also for courtship displays The flexible and telescopic abdominal segmentsallow earwigs to use their forceps in all directions and they often use this ability toassist in folding the large hindwings

The order is divided into two suborders, the Ensifera and the Caelifera.

The Ensifera, comprising the crickets and katydids, have long or very longantennae and sing by rubbing structures on their front wings together They aremainly nocturnal and solitary and most species mimic dead or living leaves.Many species are herbivorous, but some are partly or wholly predaceous Theovipositor is always prominent and sword-, sickle- or stiletto-shaped

The Caelifera, comprising grasshoppers and locusts (which show dependent polyphenism), have short antennae and the females never haveprominent ovipositors Songs are produced by a row of pegs on the hind femorarubbing against the edge of the front wings They are generally ground-living,diurnal, grass- and/or forb-feeders and can be cryptically colored or brightlycolored to advertise their unpalatability Several, such as the desert locust,Schistocerca gregaria, are serious crop pests

density-P H A S M A T O D E A

 >3000 species

The elongate body of stick insects can be short and smooth or large and veryspiny or leaf-like The head is characteristically domed and carries relativelylong, thread-like antennae, chewing mouthparts, a pair of small compound eyesand, in winged species, ocelli The front wings are short and toughened while thefan-shaped membranous hindwings are large Many species are short-winged orwingless, and in others wing length varies between the sexes

Stick insects are slow-moving, herbivorous and mostly nocturnal Their shapeand cryptic coloring make them very difficult to see among foliage and affordsthem protection from predators Some species freeze motionless when disturbed,holding the middle and hindlegs along the body and stretching out the front legs,while others sway to imitate the movement of the vegetation Leaf insects, whichare broad and flattened with fantastic leaf-like expansions, are contained in onefamily, the Phylliidae, comprising about 50 species confined to Southeast Asia,New Guinea and Australia

E M B I O P T E R A ( E M B I I D I N A , E M B I O D E A )

Webspinners are narrow-bodied, cylindrical or slightly flattened gregarious insectsliving in warm temperate and tropical regions The head has small, kidney-shapedcompound eyes, thread-like antennae and biting mouthparts The front legs of alllife-stages and both sexes have swollen basal tarsal segments containing glands,which produce silk to make communal galleries in soil, litter and under bark

As colonies grow, galleries and tunnels are extended to take in new food sourcessuch as dead plant material, litter, lichens and mosses Only adult females andnymphs feed Males do not feed as adults and only use their jaws to grasp thefemale during copulation Females are wingless but the males usually have twoequal-sized pairs of long, narrow wings The wings have hollow veins that can beinflated with hemolymph to make them stiff for flight When the veins are notinflated, the wings can fold forwards without damage when the male has to runbackwards through the galleries

Zorapterans are gregarious under bark or in piles of wood dust, leaf litter or

in termite nests, where they eat fungal threads, spores, mites and other smallarthropods As populations grow, winged morphs disperse to new locations andthe wings are then shed All the known species are currently assigned to a singlegenus, Zorotypus

P S O C O P T E R A

 <4500 species

Barklice and booklice are very common insects, which on account of their smallsize and cryptic coloration, are often overlooked The head is relatively large, withbulging compound eyes, long, thread-like antennae, biting mouthparts and, inwinged species, three ocelli The thorax is slightly humped and the wings, whenpresent, are held roof-like over the body at rest

Psocoptera can be found in a very wide range of terrestrial habitats, includingcaves and the nests of birds, bees and wasps, but are particularly abundant in litterand soil and on the bark and foliage of trees and shrubs Most species graze on algae,lichens and molds and fungal spores, but some can be pests of stored products.Three suborders are recognized – the Trogiomorpha, considered the mostprimitive, the Troctomorpha and the Psocomorpha, the most advanced suborder,containing more than 80% of the known species.

P H T H I R A P T E R A

 Parasitic lice

These small, wingless, dorso-ventrally flattened ectoparasites live permanently onbird or mammal hosts, where they feed on skin debris, secretions, feathers or blood.The eyes are very small or absent, there are no ocelli and the antennae are short,with a maximum of five segments The legs are short and robust, with the tarsi andclaws typically modified for grasping hair or feathers Several species are

significant vectors of human and animal diseases

The nymphs pass through three instars or nymphal stages, taking anything fromtwo weeks to a few months to reach adulthood Many lice have symbiotic

relationships with bacteria which live in special mycetocytes associated with thedigestive system These bacteria allow the lice to digest feather protein (keratin) andblood

There are four suborders within the Phthiraptera The Amblycera are a primitivegroup of chewing lice living on birds and mammals The Rhyncophthirina areectoparasites of elephants and warthogs The largest suborder, the Ischnocera, arechewing lice mainly found on birds, while the Anoplura are sucking lice whichinclude the human head and body louse and the pubic louse

H E M I P T E R A

 >82 000 species

True bugs range from minute, wingless scale insects to giant water bugs withraptorial front legs capable of catching fish and frogs Compound eyes are oftenprominent and ocelli may be present Bugs lack maxillary and labial palps and themandibles and maxillae, which are enclosed by the labium, take the form ofelongate, grooved stylets through which saliva can be injected and liquids sucked

up Two pairs of wings are usually present

There are four distinct suborders The Auchenorrhyncha, comprisingplanthoppers, leafhoppers, froghoppers, treehoppers, lantern bugs and cicadas, andthe Sternorrhyncha, including jumping plant lice, whiteflies, phylloxerans, aphids

and scale insects, are herbivorous The Coleorrhyncha is represented by a singlefamily of cryptic bugs found in the Southern Hemisphere The majority of speciesbelonging to the fourth suborder, the Heteroptera, are herbivorous but the subordercontains a significant number of predatory taxa and even some blood-suckingspecies A characteristic feature of heteropterans is the possession of defensive stinkglands.

Many bug species are significant plant pests and some transmit human andanimal diseases

T H Y SA N O PT E R A

Thrips are small or very small, slender-bodied insects with prominent, large-facetedeyes, short antennae and asymmetrical piercing and sucking mouthparts One mandible

is very small and non-functional while the other is sharp and stylet-like and used topenetrate plant tissue or sometimes the bodies of minute insects The other mouthpartsform hemipteran-like stylets and are used to suck up liquid food They usually havetwo pairs of very narrow, hair-fringed wings, but wings can be reduced, vestigial orabsent Three ocelli are present in winged individuals The tarsi have an eversiblebladder-like structure between the claws Many species are serious plant pests

Although these insects are most closely related to the Hemiptera, they are unusual inthat there are one or more pupa-like resting stages between the two, true nymphalstages and the adult In some cases there are three pre-adult stages of which the firstmay still be capable of feeding The next two pre-adult stages become more pupa-likewith a degree of tissue reorganization; a cocoon may even be formed

Subdivison: Holometabola

The following neopteran orders comprise the most advanced and successful of allinsects The immature stages are called larvae and look very different and havedifferent lifestyles to the adults The wings develop internally and metamorphosisfrom larva to adult takes place during a pupal stage

adult megalopterans do not feed In some male dobsonflies the jaws may beseveral times the length of the head and used in male-to-male combat or forgrasping the female Megalopterans have two pairs of similarly sized wings, whichare held roof-like over the body.

Megaloptera are found near cool, clean streams in temperate regions

Dobsonflies prefer running water while alderflies can be found in ponds and canals

as well as streams The predaceous larvae are aquatic with simple or branched,abdominal gills

Larval development can take anything from 12 months in alderflies butsometimes more than 48 months in dobsonflies Pupation takes place on landwithin a simple chamber made in moist soil, sand or mossy vegetation or underrotting wood The pupae have functional jaws, can move around freely and evenprotect themselves

to the wing margins as in neuropterans Both pairs of wings have a small, dark orpale mark called the pterostigma on the front edge, toward the wing tips Thefemales, which are a little larger than males, have a long, slender and conspicuousovipositor

Snakeflies are closely related to alderflies but differ in that the larvae arecompletely terrestrial and the adult stage feeds

of producing substances that repel some predators There are usually two pairs of

similarly sized wings held roof-like over the body at rest The venation in mostneuropterans is net-like, with the main veins forking at the wing margins.

The majority of species are predatory and mainly active in the evening or afterdark The larvae of all species, which have their mandibles and maxillae united toform a pair of sharp, sucking tubes, are highly predaceous and can be found in awide range of habitat types

The species of some families are very similar to other insects Adult Mantispidaehave enlarged, raptorial front legs like those of praying mantids Owlflies

(Ascalaphidae) are aerial predators and look very like dragonflies

C OL E O PT E R A

This very large order makes up at least 40% of all insect species

The head has conspicuous compound eyes, antennae usually with less than

11 segments and biting mouthparts Ocelli are typically absent The prothorax isusually large and freely articulated with the rest of the thorax The toughened frontwings, or elytra, meet in the body midline and cover the larger membranoushindwings, which are folded lengthwise and crosswise underneath

Beetles can be found in every conceivable terrestrial and freshwater habitat Thepossession of protective elytra has allowed beetles to burrow, dig and squeeze intoplaces that other insects cannot reach without compromising their ability to fly

The order contains many scavengers, predators and a few specialized parasites, butthe vast majority of beetle species are herbivorous and here lies the second major reasonfor their great success The rise to dominance of the flowering plants (Angiospermae) inthe Cretaceous provided herbivorous beetles with multiple opportunities to radiate

The order is divided into four very unequal suborders The Archostematacomprises a very small group of specialist wood-borers The Mxyophaga is made up

of around 60 species of small aquatic species The Adephaga, comprising about10% of all beetles, is made up of 12 families of ground-living and aquatic specieswhere the larvae and adults are predaceous The species that make up the largestsuborder, the Polyphaga, have very diverse lifestyles and eating habits

S T R E P S I P T E R A

Strepsipterans are highly specialized endoparasites of other insects in more than 30insect families belonging to the Orders Thysanura, Blattodea, Mantodea,

Orthoptera, Hemiptera, Diptera and Hymenoptera The adults are dimorphic

Females are typically endoparasitic without eyes, antennae, mouthparts, legs or

wings Males are free-living, with raspberry-like eyes, branched antennaeand wings The front wings are small and strap-like while the hindwings arefan-shaped.

The order is divided into two suborders, the Mengenillidia (one small family – theMengenillidae) and the Stylopidia (seven families) In the Mengenillidae, full-grown male and female larvae leave the hosts, which are species of silverfish, andpupation takes places outside In this family, unusually, both adult males andfemales are free-living, but the females are not grub-like and have normal features

of adult insects, such as legs and antennae Females of all other families are totallyendoparasitic They never leave the confines of their host’s body and are

surrounded by the cuticle of their own pupal stage

The legs are long and slender in most species, but in the hangingflies (Bittacidae)they are very long and used for prey capture The fifth tarsal segment of thehindlegs is enlarged and raptorial to seize small insects The majority of otherscorpionflies feed on dead or dying insects and will also feed on carrion, nectar, sapand fruit juices

S I P H O N A PT E R A

Found wherever there are suitable hosts, fleas are a distinctive and readilyrecognizable group Well over 90% of flea species feed on the blood of landmammals – the remainder are bird ectoparasites Fleas are small, wingless,tough-bodied and laterally flattened The head typically has very short,three-segmented antennae, which fit into grooves and short mouthparts forpiercing skin and sucking blood Fleas may have a pair of simple, lateral eyessimilar to ocelli, and are negatively phototactic (avoid light) The enlarged hindlegsare part of the flea’s unique and powerful jumping mechanism, which incorporates

an energy store made of a rubber-like protein called resilin

Comb-like structures on the cheeks and the posterior edge of the pronotum ofmany species, together with numerous backward-pointing spines and bristles on

the body, help the fleas to remain in the host’s fur As holometabolous insects,the larvae are very different from the adults and feed on detritus and dried blood

in the host’s lair or den Pupation takes place inside a loose cocoon Many fleaspecies are disease vectors

Flies have a mobile head with large compound eyes and three ocelli Themouthparts, which vary according to diet, are adapted for lapping and spongingliquids or piercing and sucking

A characteristic feature of the order is the possession of a single pair ofmembranous front wings, although some ectoparasitic species are wingless Thehindwings in all species are reduced to form a pair of balancing organs calledhalteres

The order is divided into two suborders, the Nematocera and the Brachycera

Nematocera is the more primitive suborder, and includes crane flies, mosquitoes,black flies, midges and fungus gnats, with delicate thread-like antennae TheBrachycera are more robust, with short, stout antennae of less than six segments, andinclude the orthorrhaphan groups, typified by horse flies and robber flies, and thecyclorrhaphan species such as fruit flies, hover flies, blowflies and flesh flies Larvalhabits vary from fully aquatic to terrestrial and many larvae are serious plant pests

or three ocelli may be present The weakly developed mouthparts allow adults

to lick up water and nectar, but many do not feed as adults The front and hindpairs of wings are held over the body in a characteristically tent-like manner

The caterpillar-like larvae, which are aquatic, show a range of feeding habits Somespecies may be free-living or spin food-catching nets, but most live inside portabletube-like cases made from sand grains, small stones or bits of vegetation heldtogether with silk secreted from glands in the head.

The order, which is most closely related to the Lepidoptera, is divided into twosuborders, the Annulipalpia, mostly with net-spinning larvae, and the

Integripalpia, comprised of species with mostly tube-case-building larvae

LE P ID O P TE R A

Members of this readily recognizable order occur everywhere there is vegetation.The body and wings of these familiar insects are covered with minute scales, whichmay be colored or iridescent The compound eyes are large and the mouthpartstypically take the form of a coiled proboscis through which liquids such as nectarcan be sucked The larvae, known as caterpillars, are typically herbivorous andhave a number of abdominal prolegs in addition to the three pairs of thoracic legs.When fully grown they spin a silk cocoon in which they pupate Some species aresignificant plant pests

The order is divided into four suborders, three of them – the Zeugloptera(one family – Micropterigidae), the Aglossata (one family – Agathiphagidae) andHeterobathmiina (one family – Heterobathmiidae) – with only a handful of species.The fourth – Glossata – contains the vast majority of the species Within theGlossata, the superfamily Papilionoidea comprises the four true butterfly families,the Papilionidae, Pieridae, Lycaenidae and Nymphalidae

The first Lepidoptera appeared in the Jurassic and then radiated greatly with therise of flowering plants in the Cretaceous

The head carries a pair of thread-like antennae, a pair of well-developedcompound eyes and, usually, three ocelli The mouthparts are adapted for chewingand biting but, in many species, liquids are ingested In the bees (superfamily

Apoidea) the maxillae and the labium are extended and modified to form a tonguethrough which nectar is sucked Most species are strong fliers with two pairs ofmembranous wings which are joined in flight by a row of small hooks calledhamuli.

The order is divided into two suborders, the Symphyta (sawflies) and the Apocrita(wasps, ants and bees) Sawflies have herbivorous larvae and the adults do not have

a constricted waist The females have a saw-like ovipositor for laying eggs intoplant tissue In the Apocrita, the first segment of the abdomen is fused to the thoraxand the second, and sometimes the third, abdominal segments are very narrow,which gives the distinctive wasp-waisted appearance Parasitic apocritans have aslender and sometimes very elongate ovipositor for penetrating and laying eggs inother insects The aculeate apocritans (stinging wasps, ants and bees) have amodified ovipositor in the form of a sting with an associated venom gland

Illustrations by Karen Hiscock-Lawrence, KHL Creative.

Part I The head, ingestion,

utilization and distribution

of food

1 Head

REVISED AND UPDATED BY STEPHEN J SIMPSON

INTRODUCTIONInsects and other arthropods are built up on a segmental plan, and theircharacteristic feature is a hard, jointed exoskeleton The cuticle, which forms theexoskeleton, is continuous over the whole of the outside of the body and consists of aseries of hard plates, the sclerites, joined to each other by flexible membranes, which are alsocuticular Sometimes the sclerites are articulated together so as to give precise movement of one

on the next Each segment of the body primitively has a dorsal sclerite, the tergum, joined

to a ventral sclerite, the sternum, by lateral membranous areas, the pleura Arisingfrom the sternopleural region on each side is a jointed appendage

In insects, the segments are grouped into three units, the head, thorax andabdomen, in which the various basic parts of the segments may be lost or greatlymodified Typical walking legs are only retained on the three thoracic segments In thehead, the appendages are modified for sensory and feeding purposes and in the abdomenthey are lost, except that some may be modified as the genitalia and in Apterygota somepregenital appendages are retained This chapter introduces the structures of the head(Section 1.1), neck (Section 1.2) and antennae (Section 1.3) Chapter 2 concerns themouthparts and feeding

The Insects: Structure and Function (5 th edition), ed S J Simpson and A E Douglas.

Published by Cambridge University Press © Cambridge University Press 2013.

1.1 Head

The insect head is a strongly sclerotized capsule

joined to the thorax by a flexible, membranous neck

It bears the mouthparts, comprising the labrum,

mandibles, maxillae and labium, and also the

antennae, compound eyes and ocelli On the outside

it is marked by grooves, most of which indicate

ridges on the inside; some of these inflexions extend

deep into the head, fusing with each other to form an

internal skeleton These structures serve to strengthen

the head and provide attachments for muscles, as

well as supporting and protecting the brain and

foregut

The head is derived from the primitive

pre-oral and post-oral segments Molecular

studies of Drosophila suggest that there are seven

head segments: labral, ocular, antennal, intercalary,

mandibular, maxillary and labial The last three

segments are post-oral and are innervated by the

three neuromeres of the subesophageal ganglion

(see Chapter 20) They are often called the gnathal

segments because their appendages form the

mouthparts of the insect The pre-oral segments are

innervated by the brain, but their nature and number

remains contentious The protocerebrum (forebrain)

innervates the compound eyes, the deutocerebrum

(midbrain) innervates the antennae, and the labrum

receives its innervation from the tritocerebrum

(hindbrain) (see Chapter 20)

1.1.1 Orientation

The orientation of the head with respect to the rest

of the body varies (Fig 1.1) The hypognathous

condition, with the mouthparts in a continuous series

with the legs, is probably primitive This orientation

occurs most commonly in phytophagous species

living in open habitats In the prognathous condition

the mouthparts point forwards and this is found in

predaceous species that actively pursue their prey,

and in larvae, particularly of Coleoptera, which use

their mandibles in burrowing In Hemiptera, the

elongate proboscis slopes backwards between theforelegs This is the opisthorhynchous condition.The mouthparts (labrum with a basal segmentcalled the clypeus, mandibles, labium and maxillae)

(b) prognathous

mandible

maxillary palp

labial palp antenna

prothoracic leg

(a) hypognathous

mandible maxillary palp labial palp antenna

prothoracic leg

mesothoracic leg

Figure 1.1 Orientation of the head (a) Hypognathous – mouthparts ventral, in a continuous series with the legs (grasshopper) (b) Prognathous – mouthparts in an anterior position (beetle larva) (c) Opisthorhynchous – sucking mouthparts with the proboscis extending back between the front legs (aphid).

enclose a cavity, the pre-oral cavity, which is divided

into two sections by the hypopharynx (Fig 1.2) The

larger of these sections, the cibarium, is enclosed

between the hypopharynx and the labrum/clypeus,

with the true mouth (the opening to the foregut) at its

inner end Between the hypopharynx and the labium

is a smaller cavity known as the salivarium, into

which the salivary duct opens

1.1.2 Rigidity

The head is a continuously sclerotized capsule with

no outward appearance of segmentation, but it is

marked by a number of grooves Most of these

grooves are sulci (singular: sulcus), marking lines

along which the cuticle is inflected to give increased

rigidity The term “suture” should be retained for

grooves marking the line of fusion of two formerly

distinct plates The groove which ends between the

points of attachment of maxillae and labium at the

back of the head is generally believed to represent the

line of fusion of the maxillary and labial segments,and is therefore known as the postoccipital suture.Since the sulci are functional mechanicaldevelopments to resist the various strains imposed

on the head capsule, they are variable in position

in different species and any one of them may becompletely absent However, the needs forstrengthening the head wall are similar in themajority of insects, so some of the sulci are fairlyconstant in occurrence and position (Fig 1.3) Themost constant is the epistomal (frontoclypeal) sulcus,which acts as a brace between the anterior

mandibular articulations At each end of this sulcus is

a pit, the anterior tentorial pit, which marks theposition of a deep invagination to form the anteriorarm of the tentorium The lateral margins of the headabove the mandibular articulations are strengthened

by a horizontal inflexion indicated externally bythe subgenal sulcus This sulcus is generally acontinuation of the epistomal sulcus to thepostoccipital suture The part of the subgenal sulcusabove the mandible is called the pleurostomal sulcus;the part behind the mandible is the hypostomalsulcus Another commonly occurring groove is thecircumocular sulcus, which strengthens the rim of theeye and may develop into a deep flange protectingthe inner side of the eye Sometimes this sulcus

is connected to the subgenal sulcus by a verticalsubocular sulcus; the inflexions associated with thesesulci act as a brace against the pull of the musclesassociated with feeding The circumantennal ridge,marked by a sulcus externally, strengthens the head

at the point of insertion of the antenna, whilerunning across the back of the head, behind thecompound eyes, is the occipital sulcus

The areas of the head defined by the sulci are givennames for descriptive purposes, but they do notrepresent primitive sclerites Since the sulci are variable

in position, so too are the areas which they delimit Thefront of the head, the frontoclypeal area, is divided bythe epistomal sulcus into the frons above and theclypeus below (Fig 1.3) It is common to regardthe arms of the ecdysial cleavage line as delimiting

frontal ganglion

brain pharynx salivary duct subesophageal ganglion suspensory sclerite salivarium

labium

lingual sclerite

Figure 1.2 Pre-oral cavity and some musculature.

Diagrammatic vertical section through the head of an insect

with biting and chewing mouthparts Sclerites associated

with the hypopharynx are black with white spots Muscles

attached to these sclerites move the hypopharynx (after

Snodgrass, 1947).

the frons dorsally, but this is not necessarily so.

From the frons, muscles run to the pharynx, the labrum

and the hypopharynx; from the clypeus arise the

dilators of the cibarium The two groups of muscles are

always separated by the frontal ganglion and its

connectives to the brain (Fig 1.2) Dorsally the frons

continues into the vertex and posteriorly this is

separated from the occiput by the occipital sulcus The

occiput is divided from the postocciput behind it by the

postoccipital suture, while at the back of the head,

where it joins the neck, is an opening, the occipital

foramen, through which the alimentary canal, nerve

cord and some muscles pass into the thorax

The lateral area of the head beneath the eyes is called

the gena, from which the subgena is cut off below by

the subgenal sulcus, and the postgena behind by the

occipital sulcus The region of the subgena above the

mandible is called the pleurostoma, and that part

behind the mandible is the hypostoma

In hypognathous insects with a thick neck, the

posterior ventral part of the head capsule is

membranous The postmentum of the labium iscontiguous with this membrane, articulating with thesubgena on either side The hypostomal sulci bendupwards posteriorly and are continuous with thepostoccipital suture (Fig 1.4a) In insects with anarrow neck, permitting greater mobility of the head,and in prognathous insects, the cuticle of the headbelow the occipital foramen is sclerotized This regionhas different origins In Diptera, the hypostomata ofthe two sides meet in the midline below the occipitalforamen to form a hypostomal bridge that iscontinuous with the postocciput (Fig 1.4b) In othercases, such as Hymenoptera and the water bugsNotonecta and Naucoris (Hemiptera), a similar bridge

is formed by the postgenae, but the bridge is separatedfrom the postocciput by the postoccipital suture(Fig 1.4c) Where the head is held in the prognathousposition, the lower ends of the postocciput fuse andextend forwards to form a median ventral plate, thegula (Fig 1.4d), which may be a continuoussclerotization with the labium Often the gula is

gena

ecdysial line

occipital sulcus

postoccipital suture

neck membrane subgenal sulcus posterior tentorial pit

(a) generalized condition (b) hypostomal bridge

hypostomal bridge

postmentum

postoccipital suture

posterior tentorial pit

hypostomal sulcus

maxilla mandible

postocciput

subgena

postgena

occipital foramen

posterior tentorial pit

hypostomal sulcus

maxilla mandible

postocciput

subgena postgena

mentum

occipital foramen

Figure 1.4 Sclerotization at the back of the head Notice the position of the bridge below the occipital foramen with

reference to the posterior tentorial pit Membranous areas stippled, compound eyes cross-hatched The names of areas defined by sulci are italicized (after Snodgrass, 1960) (a) Generalized condition, no ventral sclerotization; (b) hypostomal bridge (Deromyia, Diptera); (c) postgenal bridge (Vespula, Hymenoptera); (d) gular bridge formed from the postoccipital sclerites (Epicauta, Coleoptera).

reduced to a narrow strip by enlargement of the

postgenae and sometimes the postgenae meet in

the midline, so that the gula is obliterated The median

ventral suture which is thus formed at the point of

contact of the postgenae is called the gular suture

In all insects, the rigidity of the head is increased

by four deep cuticular invaginations, known as

apodemes, which usually meet internally to form a

brace for the head and for the attachment of muscles

The structure formed by these invaginations is called

the tentorium (Fig 1.5) Its two anterior arms arise

from the anterior tentorial pits, which in

Apterygota and Ephemeroptera are ventral and

medial to the mandibles In Odonata, Plecoptera

and Dermaptera the pits are lateral to the mandibles,

while in most higher insects they are facial at either

end of the epistomal sulcus The posterior arms arise

from pits at the ventral ends of the postoccipital

suture and they unite to form a bridge running across

the head from one side to the other In Pterygota the

anterior arms also join up with the bridge, but the

development of the tentorium as a whole is very

variable Sometimes a pair of dorsal arms arise

from the anterior arms and they may be attached

to the dorsal wall of the head by short muscles InMachilidae (Archaeognatha) the posterior bridge ispresent, but the anterior arms do not reach it, while inLepismatidae (Thysanura) the anterior arms unite

to form a central plate near the bridge and arejoined to it by very short muscles

in their development and position and, in Apterygota,they are reduced or absent The ecdysial cleavage linemay persist in the adult insect, and sometimes thecranium is inflected along this line to form a truesulcus Other ecdysial lines may be present on theventral surface of the head of larval insects

postoccipital ridge dorsal arm of tentorium

subgenal

ridge anterior arm of

tentorium

epistomal

sulcus clypeus

labrum

occiput

postocciput

postoccipital suture tentorial bridge posterior tentorial pit subgenal sulcus

anterior tentorial pit epistomal ridge

Figure 1.5 Tentorium Cutaway of the head capsule to show the tentorium and its relationship with the grooves and ridges of the head (after Snodgrass, 1935).